The Hidden Toll of Extreme Heat on Pregnancy

Ursela Salvador lives in Ngriil, a village in the small Pacific island nation of Palau, where the journey to motherhood is marked by rituals. But for most of her married life, she wasn’t sure she would ever be able to participate in them.

In the five years Salvador and her first husband tried to conceive, nothing took. By the time she married her current husband, an old friend from high school, she was 37 and had all but given up on becoming a mother. So she could hardly believe it last summer when a hospital visit for bloating revealed she would have her chance after all: Salvador was five months pregnant.

Her mother and aunts encouraged her to keep up many of the traditions they’d once observed. On their advice, she took hot showers and baths infused with local herbs. Salvador also coated her skin with coconut oil, intended to prevent melasma, the hyperpigmentation triggered by pregnancy’s hormonal changes. But while she was proud to carry on the practices of generations of Palauan women before her, some of them caused her considerable discomfort. When she was slathered in coconut oil, venturing outside gave her the sensation of being deep-fried. After years spent dreaming of motherhood, Salvador faced a threat she had never anticipated.

Palau holds the grim distinction of having the world’s highest number of pregnancy heat-risk days. The insight comes from researchers at Climate Central, a nonprofit reporting on the effects of climate change, which analyzed recent and historical temperature shifts in 247 countries and territories. Defined as days warmer than 95 percent of historical local norms (since physical and behavioral adaptations to heat vary by region), pregnancy heat-risk days are hot enough to raise the chance of preterm birth, the leading cause of neonatal mortality, among other adverse outcomes. The number of pregnancy heat-risk days is increasing globally, and in Palau, researchers recorded an average of 77 pregnancy heat-risk days annually between 2020 and 2024—all but one of which they attribute to climate change. That’s two and a half months out of the year when women and their babies are at risk.

Salvador felt the weight of that risk. When she was seven months pregnant, early contractions brought her to the hospital, where a midwife advised her to avoid being outside and to drink plenty of water, warning that early contractions could be linked to high temperatures. The midwife told Salvador a devastating story of a young pregnant woman who lost her baby after being admitted to the hospital with dehydration. “It scared me,” Salvador says.

For the rest of her pregnancy, Salvador retreated to her bedroom, curtains drawn, air conditioner on high. She was lucky; many in Palau can’t afford the cost of running one. But at 34 weeks, she noticed spots of blood in her underwear and raced to the hospital. Her contractions were already coming quickly when her baby’s heart rate dropped. She had planned for a vaginal birth, but it was too late. Salvador was rushed to a C-section just in time. Born more than a month early, her baby boy, Manasseh, spent his first four weeks in the neonatal intensive care unit outfitted with a catheter and using a CPAP device to breathe.

Now a year old, Manasseh is healthy and strong, but he was painfully fragile at birth. When Salvador was growing up, she says, “it was a rare thing to hear that there’s a premature baby.” Today she hears about it often enough to wonder if it’s becoming the norm.

Palau is far from the only place where pregnant women are at risk. As the world gets hotter, the impact of extreme heat is poised to become a pressing public health emergency. Without widespread action, millions of women like Salvador—and their babies—will pay a staggeringly high cost for a climate crisis they did little to cause. Rising temperatures have already been linked to gestational diabetes, birth defects, and even stillbirth, in addition to preterm birth, and scientists are only now beginning to understand the mechanisms by which heat endangers both mother and fetus. Meanwhile, as those scientists search for answers, the hunt for creative cooling solutions is already urgent in areas disproportionately affected by climate change.

The strain heat puts on pregnancy is something of a medical mystery. Hundreds of studies have already established that heat waves and high heat exposure increase the likelihood of adverse birth outcomes, but the precise biological triggers that lead to these outcomes are the subject of growing research.

Women’s health is among the most chronically understudied fields in medicine. As a result, our understanding of pregnancy—despite its universality—remains shockingly incomplete. Among the reasons: a historical legacy of sexism in medicine, the long-standing practice of excluding pregnant women from clinical trials, the lack of financial incentives for studying a condition that only occupies a small share of a woman’s lifespan, and the complicated ethics and politics of studying the pregnant body.

This historical neglect means that our knowledge of what happens inside a woman’s body on a hot day stems largely from old studies that never involved women in the first place. For example, a series of studies done on guinea pigs and other rodents beginning in the late 1960s became a reference point for the belief that a human maternal core temperature level of 102.2°F is dangerous for the fetus.

By the early 1980s researchers began to more fully understand that high temperatures can harm a fetus. That revelation led to warnings advising women to avoid hot tubs and saunas, especially in the first trimester, when an embryo’s cells are rapidly dividing and beginning to form the body’s organs. Given that some parts of the world now experience sauna-like temperatures for long stretches of the year, it’s imperative to get a better grasp of what exactly is going on under the hood.

Some scientists are considering whether heat-induced endocrine stress hormones cause inflammation that potentially triggers preterm birth. Others emphasize dehydration can result in lower levels of amniotic fluid and put greater pressure on the umbilical cord. Another theory centers on sweat. Pregnant women may have a lower sweat threshold than nonpregnant women, triggering cooling perspiration sooner. Since humans cool off by redirecting blood flow to the skin, this can reduce blood supply to the placenta and thus restrict nutrient delivery and waste removal from the fetus, ultimately stunting its growth.

But testing any of these hypotheses remains particularly difficult because of ethical considerations. The main one: How do you study the impact of heat on pregnancy without actually heating people up?

A group of Australian scientists believe they’ve found a path forward. The Thermal Ergonomics Laboratory at the University of Sydney houses a state-of-the-art climate chamber that allows for precise control of not just the room’s temperature but also its air circulation, simulated solar radiation, and humidity. This past summer scientists began a three-and-a-half-year study in which 270 healthy pregnant women and 90 nonpregnant women ride a stationary bike within the chamber at three different temperatures: 86°F, 95°F, and 104°F.

Pregnant women visit the chamber twice, at weeks 22 to 24 and then again at weeks 34 to 36. Nonpregnant women visit the chamber just once. All participants wear sensors to measure metrics such as their heart rate, sweat output, and skin temperature while humidity slowly ramps up. “We’ll be able to look physiologically at what happens to their energy expenditure, their core temperature, how much they sweat, what happens to the baby’s heart rate, all of those things in a very controlled environment,” says Adrienne Gordon, a clinical professor of obstetrics, gynecology, and neonatology at the University of Sydney, who is co-leading the study. The pregnant participants are then contacted at 42 weeks to track the outcome of each birth.

The study will shed light on which climatic conditions push women past a safe threshold: in this case, a body temperature of 100.4°F, midway between the baseline core temperature of 98.6°F and that dangerous threshold of 102.2°F. As soon as a participant’s temperature reaches 100.4°F, her session ends. “We’re basically comparing what the critical environmental limit is in which pregnant women can maintain their core body temperature safely and whether that differs between pregnant and nonpregnant women,” explains James Smallcombe, a senior research fellow at the University of Sydney.

Such nuanced, real-world guidance can be hard to come by when so little is known about the interaction between heat and pregnancy in the first place. When Ana Bonell, an academic clinician at the London School of Hygiene and Tropical Medicine, began her research into the topic, there was a general acceptance that women living in hot areas are acclimatized and that “their bodies are adapted so they don’t have a problem,” she recalls. “But there was no data to support that.”

Bonell recently finished gathering data from 800 pregnant women in the Gambia, many of whom work outside as subsistence farmers. They wore heart-rate-tracking devices and monitors that measured outside temperature and humidity, and their daily tasks were tracked to flag potentially high-risk activities. Nurse midwives monitored fetal growth and heart rates using portable ultrasound devices. Bonell says she expects that a comparison of these datasets will show how heat-related physiological stress in the mother “seems to transfer to the fetus.”

Samples of the women’s blood, umbilical cord blood, and placentas will be analyzed by scientists from the University of Cambridge for changes in epigenetics or placental vasculature—the intertwining maternal and fetal circulatory systems—which could help explain any differences in birth outcomes. Bonell believes that being able to measure such minute biomarker changes during pregnancy could provide useful clues about the presence of heat stress, which is especially important because public health guidance doesn’t always align with biological reality. People are advised to take certain steps when they feel unwell, explains Bonell, but “What if you have an impact on the fetus before you feel unwell? How do you know?”

In Atlanta, Georgia, affectionately referred to as Hotlanta, Emory University exposure scientist and molecular epidemiologist Donghai Liang has identified several key markers that could provide clues about when to worry. By analyzing blood samples from pregnant Black women—who, in the United States, have an average preterm birth rate of nearly 15 percent, twice the rate of white women—Liang found that high outdoor temperatures led to altered levels of molecules responsible for critical functions in a healthy pregnancy, such as the metabolism of amino acids and blood flow regulation. Those same metabolic changes were also present at altered levels in the blood of women who’d had preterm births. “That overlap is very important,” Liang says. His initial results challenge old assumptions and suggest that it doesn’t matter when during the pregnancy temperatures spike. “There doesn’t seem to be a more sensitive period of exposure,” he adds, “which tells us that maybe throughout the entire pregnancy period we need to be extremely careful.”

Another biomarker some researchers believe should be more closely and consistently tracked: heat shock proteins, or HSPs. These molecules are found in every human body and bind to vulnerable cellular proteins in order to protect them, acting as a cushion or buffer, explains Subhradip Karmakar, a biochemistry professor at All India Institute of Medical Sciences who is studying the role of HSPs in heat exposure and pregnancy outcomes. At very high levels, HSPs can actually cause collateral damage. During pregnancy, that can take the form of changing uterine blood flow, which can hinder the fetus’s ability to receive nourishment and prevent fetal heart development, leading to cardiac malformations. Karmakar has found that certain HSPs were elevated in women experiencing high heat. Testing blood for elevated levels could provide a warning “that the mother is under stress,” says Karmakar, who hopes that HSP monitoring one day becomes part of routine prenatal care.

In the meantime, there’s another urgent question to answer: What are the affordable, easily adoptable ways to find relief?

In Fort Lauderdale, Florida, where temperatures regularly go above 90°F in the summer, a new treatment strategy is being shared with one new mother at a time. The nonprofit Florida Clinicians for Climate Action has started a climate health outreach program that pairs doulas trained in heat-reduction strategies with low-income women for free home visits. They work with pregnant women to educate and prepare them for heat and climate emergencies and also help postpartum mothers caring for newborns in high temperatures.

In a small residential park just off Interstate 95, among rows of manufactured homes, many built with metal siding and lacking tree cover, a pale peach one-bedroom unit sits at the back of the lot, unshaded and exposed to the sun. There, in the room she shares with her three children and partner, Jenine Spence, 29, nurses her two-week-old with the blinds drawn. Her one-year-old daughter tumbles around on the bed, following the antics of the kids’ entertainer Ms. Rachel on television. Her five-year-old son is at school.

She is being visited by doula Latoya Evans, who presents her with a heat adaptation kit containing a handheld fan, a cooling bandanna to dip in water and wrap around the head, and a 32-ounce bottle. It also has information on nearby cooling centers, phone numbers to call in case of emergency, a sun umbrella, and a crank radio and first aid kit, since Florida is subject to frequent hurricanes. Spence also receives a dehumidifier. Participating clients typically get free portable air conditioners, but Spence fears that it would raise the electricity bill. She shares the home with another resident, who has an air conditioner. During the day, cool air circulates freely. But at night, with the resident’s door closed, the family of five, who all share one bed, are cooled by a single standing fan.

For Spence, the assistance from this relatively new program could have benefited her even earlier. She was diagnosed with preeclampsia, a serious complication characterized by high blood pressure, in two of her three pregnancies, and she experienced severe headaches in the most recent one. Her two-week-old was just four pounds, which is considered small for gestational age, when she was born via C-section. But none of Spence’s healthcare providers had ever mentioned heat as a possible cause. Learning from Evans, who is now advising her postpartum, that high temperatures could have been a factor was “a major eye-opener,” Spence says, information she wishes she had known before she became pregnant the first time. “I didn’t realize they were intertwined.”

For Brandy Sarut, 29, these services are coming just in time. South of Fort Lauderdale, in a single-story bungalow nestled amid boarded-up homes in Opa-locka, she sits with her doula, Cortés James, who works with another organization, the Southern Birth Justice Network. This is Sarut’s fifth pregnancy, and despite keeping the shades down and the air conditioner on, she is still experiencing dizziness and fainting. James runs through a list of other heat-related symptoms.

“Headaches, nausea, feeling sick?” she says. “Yup,” Sarut replies. “Swelling in the hands and feet?” “Yup.” “Trouble sleeping?” “Yup.” “Excessive sweating or feeling overheated?” “Oh, yeah.” “Muscle cramps?” “Definitely.” They agree that Sarut will upgrade the small pink cup she uses for ice water to the large tumbler that comes with the heat adaptation kit, adding lemon or lime juice and a touch of sugar for hydration.

After James shows her the kit, they walk through the bedrooms to inspect Sarut’s home, making sure there aren’t cracks around the windows or other places where heat can sneak in. James peeks into the bathroom and confirms it is well ventilated. But there are other risks. Sarut can’t avoid the heat when she has to take her kids to and from school or to the pediatrician. The family shares one car, and its air conditioner blew out just a few days after they got it. “I sweat and sweat and sweat and burn up the whole day,” Sarut says, displaying her sunburned arms. “I can’t deal with it.”

Similar assistance programs are underway elsewhere. Temperatures across southern Africa are rising twice as rapidly as the rest of the globe, and a collaborative research project in Zimbabwe and South Africa is testing cost-effective, multipronged interventions that can help women at home, at work, and in their communities. These include personal heat care packages, access to cooling hubs, and tree planting or reflective roof painting to keep heat from being absorbed.

In India, some expectant mothers can avoid heat risks thanks to economic support. There, self-employed women can join a program that offers heat insurance to help protect pregnant women who work outside. When satellite data register a threshold temperature, the system triggers a payout so that women can skip work on the hottest days.

At the same time, many global health groups are building automatic alert systems with advisories and tips. A project from Heat Indicators for Global Health Horizons, a collaborative research consortium spanning 10 countries in Europe and sub-Saharan Africa, is rolling out an app with weather alerts translated into local languages and easy-to-follow guidance, like reminders that babies don’t need to wear clothing when it’s hot out or that moms can find relief from the heat by putting their feet in water.

The need for cooling solutions is only going to grow. Extreme heat is already a reality for people in nearly every corner of the globe, but its burden is borne unequally. Geography matters: Island nations like Palau are at the leading edge of the rise in extreme heat days. Researchers have also calculated a 45 percent increase in the risk of preterm birth from extreme heat exposure in lower-middle-income countries than in high-income ones. Vulnerability is further shaped by factors such as one’s built environment, access to things like shade, clean water, and nutritious food, and local availability and training of health providers who can recognize and treat heat-related illnesses.

In a new study, environmental health scientist and community advocate Hasibe Caballero-Gómez is looking at thermal inequities in Los Angeles County, California, through the view of a pregnant woman’s exposure to extreme heat in her daily life. Caballero-Gómez does this by estimating the risk of exposure to high temperatures within built environments (building heights, amount of asphalt, and tree canopy coverage), neighborhoods, workplaces, and homes—specifically how long it takes for homes to reach indoor temperatures above 90°F. “The same people who have hot homes also have hot jobs and also have hot neighborhoods,” she says. Next she compares those results against five years of birth records. She has found that especially risky preterm births—babies who are born at 22 to 27 weeks—are more common among women who are vulnerable to high heat at home, work, and in their neighborhood, and also women who experience an extreme heat event the week before delivery.

What’s more, Caballero-Gómez has discovered that both people of color and economically disadvantaged groups are “facing the brunt of exposure.” Birth parents who are Black, Latino, Mexican immigrants, or less formally educated have consistently higher likelihoods of heat exposure.

For all we’re learning about who is most at risk and how they’re affected, one looming question remains: How will this knowledge, as it unfolds, influence the very decision to have a child?

Some people already cite climate anxiety as a factor deterring them from having children. “At what temperature do the adverse outcomes of pregnancy rise to a level that people will just not do it?” Bonell wonders. When will the worsening odds dissuade them from attempting parenthood at all?

This is a consideration that haunts Ursela Salvador as she thinks about her own family’s future. She and her husband live with her parents and have a comfortable life. They can afford air-conditioning. They have a wide support system and access to healthcare. Salvador spends much of her time at home playing with Manasseh and, naturally, is besotted with him. His babbling is so lilting that Salvador can’t tell whether he is trying to talk or sing. She laughs as she describes his antics: careening around the house in his walker chasing the family’s cat and dog, climbing on top of all his pillows, and attempting to stand up. But Salvador isn’t sure that she could handle the heat, the risk, or the harrowing scramble of an emergency C-section again.

“I want another baby,” she says. “But I’m scared.”